Oil and gas wells are typically drilled by installing a conductor pipe to first depth, then drilling the well to a second depth. Normally, a string of casing is made up by coupling together sections of pipe, each being about forty feet long, and lowering the string through the conductor pipe to the second depth. Cement is then pumped down the casing, which flows back up the annulus between the casing and the open borehole. Drilling is resumed to a third depth and the process is repeated with another smaller diameter nested casing. An even smaller diameter string of casing may be installed to a fourth depth.
Casing serves to support the borehole wall and to prevent undesired outflow of drilling fluid into the formation or inflow of fluid from the formation into the borehole from strata other than the target production strata. The nested arrangement of successively smaller casing strings requires a relatively large borehole at the upper part of the wellbore due to the thickness of the casing and couplings and also due to the minimum clearance necessary between casing to displace cement in the annulus space.
Large diameter wells are more expensive to drill since they request larger drillbit, more mud and more cutting disposal. Also, a casing with a large diameter has a lower operating pressure for the same wall thickness than a smaller diameter casing. Consequently, each casing may have to cover the preceding one up to the wellhead to enhance the well pressure capability, as the well goes deeper.
Whenever pressure is not critical for the production casing, which is the last casing lowered and in consequence the smallest in diameter, the last casing is run as liner of the preceding casing and thus instead of extending it up to the wellhead, the top of a liner is suspended from the inside surface of the preceding casing, generally at an elevation right above the shoe of the preceding casing. A certain length of casing is saved, also increasing the higher section of the well, but since the liners are always formed by the assembly of pipes of a diameter lower than the preceding casing, a bore restriction on a substantial length between the production zone and the wellhead still exists.
Lastly, today when a well is drilled in a unknown zone (exploratory drilling) it is necessary to increase the amount of casing in order to be able to face the possibility to have to case the well before having reached the targeted depths. To take care of this kind of difficulty, an extra casing is considered which requires to start to drill from the surface with the next size above what would be usually needed, with a substantial increase in the cost of exploratory drilling.